1. Field of the Invention
The present invention relates to a tractor having draft controlling means configured to execute a draft control of a ground work implement during a work by this ground work implement liftably mounted to a traveling machine body which travels by a traveling apparatus, the draft control being executed based on the draft load value detected by a draft load sensor.
2. Description of the Related Art
In the Japanese Unexamined Patent Application Publication No. 5-137406 (JP 5-137406 A) having the above-described arrangement, there is described a construction as follows. Namely, a detecting means is provided for detecting respective rotations of right and left front wheels of a four-wheel drive type tractor. From detection results of this detecting means and based on a rotational speed difference between the right and left wheels (a slip ratio), a slip calculation value is obtained. In order to keep this slip calculation value below a set value, there is provided a forcible elevation controlling means for causing a plow work implement to move up, with priority over the draft control, when this slip calculation value exceeds the set value.
According to JP 5-137406 A, when the draft load of the plow work implement is large, there is exerted a moment toward the front rising side of the machine body, so that a slip tends to occur with reduction in the ground contact load of the front wheels, which slip causes in turn a rotational speed difference between the right and left front wheels. Then, upon recognition of occurrence of slip due to this rotational speed difference, the work implement is forcibly elevated, thereby to reduce the draft load, so that occurrence of slip can be restricted.
Further, in the arrangement disclosed in the U.S. Pat. No. 3,913,680, based on a rotational speed difference between a freely rotated wheel and a drive wheel, a slip ratio is obtained. And, during a draft control, in order to keep the slip ratio at a predetermined value, a solenoid valve is controlled to lift up/down the implement.
When a ground work implement providing a large draft load such as a plow is employed and a plowing work is effected with a draft control, there is executed a control wherein as the plow is elevated in association with increase in the draft load, increase of the draft load is restricted and the plow is lowered in association with decrease in the draft load.
However, if slip occurs in a wheel when this wheel is receiving a draft load from the plow, there occurs reduction in the draft load, so that a control operation for lowering the plow may be executed inadvertently. Hence, there may occur a situation when the plowing work cannot be continued since the slip is not solved. When such situation as this not allowing continuation of the plowing work has developed, the worker can easily recognize this. However, there may occur a phenomenon of the wheel slipping for a short period in repetition during traveling. In such case like this, the working efficiency may sometimes deteriorate as the worker continues to fail to recognize the occurrence of slip.
To cope with such inconvenience as above, it is conceivable to effect a control operation of forcibly elevating the ground work implement based on wheel slip, as described in JP 5-137406 A and U.S. Pat. No. 3,913,680. As such control mode for forcibly lifting up or elevating the ground work implement, it is conceivable for example to provide a hydraulic cylinder for lifting up/down the ground work implement and a control valve for effecting feeding/discharging of working oil to/from this hydraulic cylinder, so that when the slip ratio is below a threshold value, the control valve is operated based on the draft load for draft control, whereas when the slip ratio exceeds the threshold value, the greater the value exceeding the threshold value, the greater the opening ratio of the control value, for operating the hydraulic cylinder for its rising direction. However, with such mode of control, in case the slip ratio has exceeded the threshold value significantly, as the large opening ratio is set for the control value, the hydraulic cylinder will be operated in the rising direction at a high speed, whereby the slip will be solved in a short period time in association with high speed rising movement of the ground work implement. Nevertheless, as no rising target is set for the ground work implement, the ground work implement will be elevated more than necessary and it will take time until the implement returns to the target plowing depth after this rising movement, so that a condition of excessively shallow plowing depth may continue for some time disadvantageously. In this respect, there remains room for improvement.
The object of the present invention is to reasonably construct a tractor capable of causing the ground work implement to rise up in the event of slip of traveling apparatus to allow continuation of the ground work.